WTG cloud modeling with spectral decomposition of heating

A modified form of the relaxed weak temperature gradient (WTG) approximation is described and implemented in a cloud resolving model. In this scheme, the vertical heating profile is spectrally decomposed into normal modes. The relaxation time scale of each mode is set proportional to the corresponding vertical wave number of the mode, as would be expected if gravity waves were responsible for the relaxation. Buoyancy adjustment exhibits greater realism than in conventional relaxed WTG, with nonlocal adjustment occurring as in the weak pressure gradient model of Romps. Furthermore, no boundary layer taper is needed when using the improved scheme. Experiments from the literature are revisited; mass flux profiles are smoother and multiple equilibrium experiments are more robust.